Grey colored glass is desirable in certain architectural applications, for instance where the glass when used in window applications protects the occupants or interior of a building from ambient solar conditions. Additionally, the automotive industry, for a number of years, has desired glass having grey color for automotive window applications. At the same time, it is also desirable for transmission in the IR (infrared) range of the light spectrum to be minimized. It is difficult to couple desirable grey color, high visible transmittance (e.g., 55% or higher), and reduced IR transmittance in a commercially acceptable glass.
A glass window or other glass article is said to have the desirable color “grey” when it has a dominant wavelength of from 435 nm to 570 nm, more preferably from 470-555 nm, and most preferably from about 480-500 nm. Moreover, grey glass preferably has an excitation purity (Pe) of less than or equal to about 5.0%.
While glass having “grey” color is often desirable, as explained above there sometimes also exists a need or desire to achieve certain levels of light transmission defined conventionally by:                Lta as visible light transmission,        UV as ultraviolet light transmission, and        IR as infrared light transmission.        
Glass thickness ranges of from about 1-7 mm, more preferably from about 3-4 mm, are typically used when measuring the aforesaid characteristics. These thickness ranges are generally recognized as conventional thicknesses for glass sheets made by the float glass process, as well as recognized thickness ranges in the automotive industry.
Certain known green solar control float glasses are formulated so as to achieve desirable solar characteristics due in large part to their use of large quantities of total iron. Unfortunately, the green coloration of such glasses does not always harmonize well with certain exterior automotive paints and sometimes affects vehicle interiors when viewed through the glass, and large amounts of iron are not always desirable for glass processing.
U.S. Pat. No. 6,235,666 discloses a grey glass composition capable of achieving good solar performance characteristics, including the desirable color grey. In particular, U.S. Pat. No. 6,235,666 discloses a grey glass with a colorant portion including 0.5-0.8% total iron (expressed as Fe2O3), 0.5-3.0% Er2O3, and 0.0-1.0% TiO2. While this is an excellent glass, it is sometimes undesirable in that it requires much of the very expensive erbium oxide (Er2O3). Thus, there exists a need in the art for a grey glass which can achieve desired grey color in combination with acceptable solar performance properties, without the need for much erbium.
U.S. Pat. No. 7,135,425 also discloses a grey glass composition. Example 1 of the '425 patent includes 0.28% total iron (expressed as Fe2O3), 0.18% erbium oxide, 3 ppm Se, 19 ppm cobalt oxide, 0.145% FeO; and a glass redox of 0.5, thereby achieving a visible transmission of about 70.5%, IR transmittance (% IR) of about 40.3, and total solar transmittance (% TS) of about 53.07. The '425 patent explains that IR absorption is achieved by using a high glass redox, namely of at least 0.35, and preferably at least 0.46 (e.g., see column 4, lines 60-66). Unfortunately, while such glasses of the '425 patent achieve good color and are acceptable in many respects, they are undesirable with respect to requiring such high glass redox values (i.e., glass redox of greater than 0.35, preferably at least 0.46). Such high glass redox values are undesirable in that blue coloration can result therefrom so that true grey color cannot be achieved; the '425 patent uses the expensive element erbium to compensate for this effect. Additionally, such high glass redox values are undesirable in that they require special compensation during the glass manufacturing such as furnace atmosphere compensation and/or significant refining additions to the batch mixture as will be appreciated by those skilled in the art.
U.S. Pat. No. 5,364,820 discloses a neutral grey glass. Example 1 of the '820 patent includes, for example, 0.403% total iron (expressed as Fe2O3), 0.41% cerium oxide, 0.31% titanium oxide, 23.2 ppm CoO, 7.6 ppm Se, and a glass redox of 0.243. This example of the '820 patent has a visible transmission of 70.3%, a total solar transmission (% TS) of 60.4%, and an infrared (IR) transmission (% IR) of 59%. Unfortunately, this example of the '820 patent is undesirable due to its very high IR transmittance (% IR) and also its very high amount of expensive cerium oxide required. In particular, it is often undesirable to allow this much IR radiation through the glass, and it is also undesirable to require this much expensive cerium oxide.
In view of the above, it is apparent that there exists a need in the art for a new glass composition which overcomes one or more of the above problems while achieving desired grey color and desired solar management property(ies) (e.g., IR blocking functionality) of the particular industry in which it is to be used.